Store fewer orphan tx by default, add -maxorphantx option

There is no reason to store thousands of orphan transactions;
normally an orphan's parents will either be broadcast or
mined reasonably quickly.

This pull drops the maximum number of orphans from 10,000 down
to 100, and adds a command-line option (-maxorphantx) that is
just like -maxorphanblocks to override the default.

src/init.cpp

@@ -207,6 +207,7 @@ std::string HelpMessage(HelpMessageMode hmm)
     strUsage += "  -dbcache=<n>           " + strprintf(_("Set database cache size in megabytes (%d to %d, default: %d)"), nMinDbCache, nMaxDbCache, nDefaultDbCache) + "\n";
     strUsage += "  -loadblock=<file>      " + _("Imports blocks from external blk000??.dat file") + " " + _("on startup") + "\n";
     strUsage += "  -maxorphanblocks=<n>   " + strprintf(_("Keep at most <n> unconnectable blocks in memory (default: %u)"), DEFAULT_MAX_ORPHAN_BLOCKS) + "\n";
+    strUsage += "  -maxorphantx=<n>       " + strprintf(_("Keep at most <n> unconnectable transactions in memory (default: %u)"), DEFAULT_MAX_ORPHAN_TRANSACTIONS) + "\n";
     strUsage += "  -par=<n>               " + strprintf(_("Set the number of script verification threads (%u to %d, 0 = auto, <0 = leave that many cores free, default: %d)"), -(int)boost::thread::hardware_concurrency(), MAX_SCRIPTCHECK_THREADS, DEFAULT_SCRIPTCHECK_THREADS) + "\n";
     strUsage += "  -pid=<file>            " + _("Specify pid file (default: bitcoind.pid)") + "\n";
     strUsage += "  -reindex               " + _("Rebuild block chain index from current blk000??.dat files") + " " + _("on startup") + "\n";

src/main.cpp

@@ -3819,7 +3819,8 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv)
             AddOrphanTx(tx, pfrom->GetId());
 
             // DoS prevention: do not allow mapOrphanTransactions to grow unbounded
-            unsigned int nEvicted = LimitOrphanTxSize(MAX_ORPHAN_TRANSACTIONS);
+            unsigned int nMaxOrphanTx = (unsigned int)std::max((int64_t)0, GetArg("-maxorphantx", DEFAULT_MAX_ORPHAN_TRANSACTIONS));
+            unsigned int nEvicted = LimitOrphanTxSize(nMaxOrphanTx);
             if (nEvicted > 0)
                 LogPrint("mempool", "mapOrphan overflow, removed %u tx\n", nEvicted);
         }

src/main.h

@@ -44,8 +44,8 @@ static const unsigned int DEFAULT_BLOCK_PRIORITY_SIZE = 50000;
 static const unsigned int MAX_STANDARD_TX_SIZE = 100000;
 /** The maximum allowed number of signature check operations in a block (network rule) */
 static const unsigned int MAX_BLOCK_SIGOPS = MAX_BLOCK_SIZE/50;
-/** The maximum number of orphan transactions kept in memory */
-static const unsigned int MAX_ORPHAN_TRANSACTIONS = MAX_BLOCK_SIZE/100;
+/** Default for -maxorphantx, maximum number of orphan transactions kept in memory */
+static const unsigned int DEFAULT_MAX_ORPHAN_TRANSACTIONS = 100;
 /** Default for -maxorphanblocks, maximum number of orphan blocks kept in memory */
 static const unsigned int DEFAULT_MAX_ORPHAN_BLOCKS = 750;
 /** The maximum size of a blk?????.dat file (since 0.8) */